Dissociation and solubility variation versus pO2- of scheelite CaWO4 in molten NaCl−KCl (At 1000 K)

Abstract

The conditional solubility of scheelite CaWO 4 in molten NaCI--KC1 (1 : 1) has been studied either in oxobasic or oxoacidic media. In the former case it is increased by formation of sparingly soluble CaO according to: Ca 2+ + 02- ~ CaO(s), pK Ca° = 10800/T-- 5.8 (molality scale) In the later case, WO 2- behaves as an oxobase according to the following equilibria: WO42- ~- WO 3 (s) + 02_ pK 0 = 10.0 at 1000 K 3 WO2--¢- W30120+ 02_ pK 3 = 12.7 at 1000 K The latter equilibrium favours an increase in the W VI solubility when pO 2- is increased, for instance by injection of HC1. The whole set of results has been summarized by a conditional solubility diagram of scheelite versus pO 2-.